Certain powersplit transmissions suffer from their associated power sources, such as an internal combustion engine, stalling when the vehicle encounters a load, such as when the vehicle enters a pile of aggregate material. The vehicle may be, by way of example, a wheel loader. In most cases, the stall condition occurs when the transmission is in a driving gear configuration used for higher speeds.
Typically, the stall occurs because the output of the transmission maintains driving engagement with the power source and the overall transmission ratio set cannot follow the fast deceleration (speed gradient) that the vehicle is subjected to when entering a pile. At a certain point, the power source is unable to deliver the requested drive torque, which results in an engine stall.
In a wheel loader equipped with a conventional powersplit transmission, this rapid changing ratio set is in the first instance accomplished by the torque converter. When the wheel loader is entering a pile (typically in a driving gear configuration used for higher speeds) the speed ratio of the torque converter changes from a high value (for example approximately 0.8 to approximately 0.9, which is typically associated with a driving mode) to a rather low value, or even zero (stall condition).
Another advantage of the torque converter is that the torque received from the engine is increased during the stall condition with a factor (typically a factor of 2 to 3 in these applications), enhancing the push power of the machine. To even further increase the push power of the machine, a downshift may be made from the second driving gear to the first driving gear to obtain a maximum push power available from the vehicle. This downshift typically occurs with an interruption in the torque delivered by the vehicle.
In order to overcome the above-mentioned deficiency, the transmission may be put into neutral. The vehicle thus comes to a full stop, such as during pile entry. Re-initialization of the drive train is then required prior to resuming the actual operation. This solution significantly compromises the performance of the wheel loader to a negative extent and therefore is unacceptable for modern vehicles. Generally, transmissions convert power from the power source, for example a diesel engine, to the drive axles of the vehicle. Some transmissions have a powersplit configuration. This means that power can be transferred by using a hydraulic branch, a mechanical branch or a combination of both branches used together.
Powersplit transmissions can be operated in different modes. In a first drive range, the transmission is operated in the hydrostatic mode, where only a hydrostatic motor drives an output of the vehicle. In a second or a third drive range, the transmission is operated in the powersplit mode, where a combination of the hydraulic branch and the mechanical branch of the transmission are used to drive the output of the vehicle.
It would be advantageous to develop a method of operating a driveline including a power split transmission that militates against a torque interruption from occurring during a shifting procedure and militates against a stall condition from occurring.